Speaking from experience, those are business practice problems, not technical competence issues.

You do get what you pay for, but top line (counting the middlemen on both sides) Eastern European outsorcing rates are only about ~30% lower than US rates these days, and people still think of it as a cheap labor destination. So companies give you 25% allocation while pretending to give you 100% and such to make the math work out. Lots of shady business practices like that + outsorcing companies don't really give a damn about your product. I imagine you're thinking startups since we're talking "apps" here, and the industry gameplan there has been to bleed them dry for a while now unfortunately.

But if you're outstaffing and can actually manage the talent yourself, trust me these guys have no issue going toe-to-toe with US devs.

You bring up a valid point about why though (despite bad comp). My guess is free education up to and including your PhD, general technical inclination, differences in values (a lot of them straight up refuse to move or change their lifestyle for 4x the money for instance, almost inconceivable in the US). I do wonder if that will last though.

Of course it really depends on who you hire, there are also shit developers everywhere and you can get majorly screwed if you don't know what you're doing, and that becomes way more likely the moment you're hiring abroad (information asymmetry is a removed).